1. Field of the Invention
The invention relates to a method of measuring fluid flow rate in a system in which fluid flows via a conduit from one location therein to another. In particular, the method is advantageously used in measuring flow rate within the cardiovascular system of a living being.
2. Prior Art
One of the key cardiovascular parameters used by the cardiologist to diagnose and follow the progress of acutely ill cardiac patients is their cardiac output. Key to their treatment is the knowledge of this parameter. Cardiac output is generally measured using some form of dye dilution technique. For example, the "Fick" method measures the breathing rate and determines oxygen uptake as well as both venous and arterial blood oxygen level. The cardiac output can then be computed based on the sample values.
A number of flow rate measuring methods operate on the indicator-dilution-principle. For example, cold blood or a dye, may be injected, via a catheter inserted from downstream of the heart, backwardly therethrough and into the superior vena cava, from whence it will flow through the right atrium and right ventricle to the main pulmonary artery. In the case of cold blood, a pair of thermistors, or other temperature sensing instruments, are attached to the catheter, one slightly downstream of the point of injection and the other further downstream, generally corresponding to somewhere in the main pulmonary artery. A temperature versus time measurement at the first thermistor shows a sharp pulse, whereas, a temperature versus time measurement at the second thermistor shows a diffuse or spread out pulse. Through integration of the pulses, a measurement of blood flow through the heart (cardiac output) is thereby obtained. Similarly, the flow of a dye can be observed, e.g., using fiber optics, and correlated to cardiac output. A description of the theory behind the indicator-dilution method appears in the Handbook of Physiology-Circulation, Volume I, Chapter 18 at pages 585-591, American Physiologic Society, Washington D.C., 1962 and in Dye Curves D. A. Bloomfield, University Park Press, Baltimore, 1974.
The indicator-dilution methods of the prior art have required inserting catheters upstream through the heart with thermistors or dye flow detectors attached thereto, thus causing patient discomfort and, possibly traumatic shock. A finite risk is associated with routine usage of this procedure. In some patients such insertion is impossible because of the particular nature of the patient's heart valves which may not be known ahead of time. Further, the prior are methods have been subject to possible error due to somewhat uncertain positionings of the catheters and observations of the dye flow.
It would be advantageous to have an indicator-dilution method which did not suffer from the aforementioned problems. In particular, it would be advantageous to provide an indicator-dilution method wherein catheter insertion starting downstream of the heart and continuing therethrough was not necessary, and where the only invasion of the body itself was via injection of the indicator well upstream of the heart, for example, into the median cubital vein adjacent the elbow. It would also be advantageous if accurate measurement techniques for the flow of the indicator were to form a part of such a method, whereby possible operator error was eliminated or at least minimized.